Apache mod_rewrite Flags

This document discusses the flags which are available to the
RewriteRule directive,
providing detailed explanations and examples. This is not necessarily
a comprehensive list of all flags available, so be sure to also
consult the reference documentation.

Each flag has a long and short form. While it is most common to use
the short form, it is recommended that you familiarize yourself with the
long form, so that you remember what each flag is supposed to do.

Presented here are each of the available flags, along with an example
of how you might use them.

The [C] or [chain] flag indicates that the RewriteRule is chained to the next
rule. That is, if the rule matches, then it is processed as usual and
control moves on to the next rule. However, if it does not match, then
the next rule, and any other rules that are chained together, will be
skipped.

The [CO], or [cookie] flag, allows you to set a cookie when a
particular RewriteRule
matches. The argument consists of three required fields and two optional
fields.

You must declare a name and value for the cookie to be set, and the
domain for which you wish the cookie to be valid. You may optionally set
the lifetime of the cookie, and the path for which it should be
returned.

By default, the lifetime of the cookie is the current browser
session.

By default, the path for which the cookie will be valid is "/" - that
is, the entire website.

This rule doesn't rewrite the request (the "-" rewrite target tells
mod_rewrite to pass the request through unchanged) but sets a cookie
called 'frontdoor' to a value of 'yes'. The cookie is valid for any host
in the .apache.org domain. It will be set to expire in 1440
minutes (24 hours) and will be returned for all URIs.

With the [E], or [env] flag, you can set the value of an environment
variable. Note that some environment variables may be set after the rule
is run, thus unsetting what you have set. See the
Environment Variables document for more details on how Environment
variables work.

The following example sets an evironment variable called 'image' to a
value of '1' if the requested URI is an image file. Then, that
environment variable is used to exclude those requests from the access
log.

Using the [F] flag causes Apache to return a 403 Forbidden status
code to the client. While the same behavior can be accomplished using
the Deny directive, this
allows more flexibility in assigning a Forbidden status.

The following rule will forbid .exe files from being
downloaded from your server.

RewriteRule \.exe - [F]

This example uses the "-" syntax for the rewrite target, which means
that the requested URI is not modified. There's no reason to rewrite to
another URI, if you're going to forbid the request.

The [L] flag causes mod_rewrite to stop processing
the rule set. In most contexts, this means that if the rule matches, no
further rules will be processed.

If you are using RewriteRule in either
.htaccess files or in
<Directory> sections,
it is important to have some understanding of how the rules are
processed. The simplified form of this is that once the rules have been
processed, the rewritten request is handed back to the URL parsing
engine to do what it may with it. It is possible that as the rewritten
request is handled, the .htaccess file or
<Directory> section
may be encountered again, and thus the ruleset may be run again from the
start. Most commonly this will happen if one of the rules causes a
redirect - either internal or external - causing the request process to
start over.

It is therefore important, if you are using RewriteRule directives in one of these
context that you take explicit steps to avoid rules looping, and not
count solely on the [L] flag to terminate execution of a series of
rules, as shown below.

The example given here will rewrite any request to
index.php, giving the original request as a query string
argument to index.php, however, if the request is already
for index.php, this rule will be skipped.

The [N] flag causes the ruleset to start over again from the top. Use
with extreme caution, as it may result in loop.

The [Next] flag could be used, for example, if you wished to replace a
certain string or letter repeatedly in a request. The example shown here
will replace A with B everywhere in a request, and will continue doing
so until there are no more As to be replaced.

RewriteRule (.*)A(.*) $1B$2 [N]

You can think of this as a while loop: While this
pattern still matches, perform this substitution.

By default, special characters, such as & and
?, for example, will be converted to their hexcode
equivalent. Using the [NE] flag prevents that from happening.

RewriteRule ^/anchor/(.+) /bigpage.html#$1 [NE,R]

The above example will redirect /anchor/xyz to
/bigpage.html#xyz. Omitting the [NE] will result in the #
being converted to its hexcode equivalent, %23, which will
then result in a 404 Not Found error condition.

Use of the [NS] flag prevents the rule from being used on
subrequests. For example, a page which is included using an SSI (Server
Side Include) is a subrequest, and you may want to avoid rewrites
happening on those subrequests.

Images, javascript files, or css files, loaded as part of an HTML page,
are not subrequests - the browser requests them as separate HTTP
requests.

Use of the [P] flag causes the request to be handled by
mod_proxy, and handled via a proxy request. For
example, if you wanted all image requests to be handled by a back-end
image server, you might do something like the following:

RewriteRule (.*)\.(jpg|gif|png) http://images.example.com$1.$2 [P]

Use of the [P] flag implies [L] - that is, the request is immediatly
pushed through the proxy, and any following rules will not be
considered.

The target (or substitution string) in a RewriteRule is assumed to be a
file path, by default. The use of the [PT] flag causes it to be treated
as a URI instead. That is to say, the
use of the [PT] flag causes the result of the RewriteRule to be passed back through
URL mapping, so that location-based mappings, such as Alias, for example, might have a chance to take
effect.

If, for example, you have an
Alias
for /icons, and have a RewriteRule pointing there, you should
use the [PT] flag to ensure that the
Alias is evaluated.

When the replacement URI contains a query string, the default behavior
of RewriteRule is to discard
the existing query string, and replace it with the newly generated one.
Using the [QSA] flag causes the query strings to be combined.

Consider the following rule:

RewriteRule /pages/(.+) /page.php?page=$1 [QSA]

With the [QSA] flag, a request for /pages/123?one=two will be
mapped to /page.php?page=123&one=two. Without the [QSA]
flag, that same request will be mapped to
/page.php?page=123 - that is, the existing query string
will be discarded.

Use of the [R] flag causes a HTTP redirect to be issued to the browser.
If a fully-qualified URL is specified (that is, including
http://servername/) then a redirect will be issued to that
location. Otherwise, the current servername will be used to generate the
URL sent with the redirect.

A status code may be specified, in the range 300-399, with a 302 status
code being used by default if none is specified.

You will almost always want to use [R] in conjunction with [L] (that is,
use [R,L]) because on its own, the [R] flag prepends
http://thishost[:thisport] to the URI, but then passes this
on to the next rule in the ruleset, which can often result in 'Invalid
URI in request' warnings.

The [S] flag is used to skip rules that you don't want to run. This
can be thought of as a goto statement in your rewrite
ruleset. In the following example, we only want to run the RewriteRule if the requested URI
doesn't correspond with an actual file.

This technique is useful because a RewriteCond only applies to the
RewriteRule immediately
following it. Thus, if you want to make a RewriteCond apply
to several RewriteRules, one possible technique is to
negate those conditions and use a [Skip] flag.

Sets the MIME type with which the resulting response will be
sent. This has the same effect as the AddType directive.

For example, you might use the following technique to serve Perl
source code as plain text, if requested in a particular way:

# Serve .pl files as plain text
RewriteRule \.pl$ - [T=text/plain]

Or, perhaps, if you have a camera that produces jpeg images without
file extensions, you could force those images to be served with the
correct MIME type by virtue of their file names:

# Files with 'IMG' in the name are jpg images.
RewriteRule IMG - [T=image/jpg]

Please note that this is a trivial example, and could be better done
using <FilesMatch>
instead. Always consider the alternate
solutions to a problem before resorting to rewrite, which will
invariably be a less efficient solution than the alternatives.

If used in per-directory context, use only - (dash)
as the substitution for the entire round of mod_rewrite processing,
otherwise the MIME-type set with this flag is lost due to an internal
re-processing (including subsequent rounds of mod_rewrite processing).
The L flag can be useful in this context to end the
current round of mod_rewrite processing.